Enlarging and spatially equalizing a zone of quiet by using active control of sound at virtual microphones

Active noise control systems, which use a microphone measurement in order to update the control signal accordingly result in reducing sound pressure level at the microphone location and a close irregular vicinity around the microphone. It is said that a local zone of quiet is then generated. However, distribution of the reduction levels is very irregular both inside and outside the zone. Dependent on many factors, and mostly on signal frequencies, the reduction may be very high at some small-size areas, then drop rapidly, and after a few centimeters the noise may be reinforced. The system performing in such way is not accepted for most applications. A remedy to this problem is to use a number of closely spaced error microphones, and minimize sound pressure levels around each of them. However, using a number of real microphones makes the arrangement not ergonomic and difficult to use. In this paper a virtual microphone control system is designed in order to enlarge the zone of quiet and reduce spatial gradient of the sound pressure level. Instead of minimizing signals acquired by a number of real microphones, estimated signals of a number of virtual microphones are minimized, in the square sense. Additional benefits of the proposed system are also emphasized. They include reduced unpleasant transient acoustic effects during adaptation, not necessity to model the primary noise at the virtual locations, and others. Performance of such system is experimentally verified.